1. Field of the Invention
The present invention relates to an electrophotographic (EP) photoreceptor within an EP printer. More particularly, the invention relates to an improved method for the production of triarylamine compounds comprising two alkyl alcohol groups. The resulting triarylamine alkyl alcohols are important for use as the starting materials for synthesis of triarylamine compounds comprising two crosslinkable group.
2. Description of the Related Art
An EP printer, such as a laser printer, is comprised of a print engine and a replaceable EP process cartridge. The replaceable process cartridge supplies toner, as well other wearing components necessary for the electrophotographic process. The photoreceptor is generally thought of as a replaceable supply item, but may be found within either the print engine, or in the process cartridge.
In electrophotography, a latent image is created on the surface of an imaging member such as a photoconducting material by selectively exposing areas of the surface to light. A difference in electrostatic charge density is created between those areas on the surface which are exposed to light and those areas on the surface which are not exposed to light. The latent electrostatic image is developed into a visible image by electrostatic toners. The toners are selectively attracted to either the exposed or unexposed portions of the photoconductor surface, depending on the relative electrostatic charges on the photoconductor surface, the development electrode and the toner.
Although organic electrophotographic photoconductors may be of single layer construction, many organic photoconductors have a dual layer construction. Dual layer photoconductors typically comprise a substrate such as a metal ground plane member on which a charge generation layer and a charge transport layer are coated. When the charge transport layer is formed on the charge generation layer, the photoconductor exhibits a negative charge on its surface. Conversely, when the charge generation layer is formed on the charge transport layer, the photoconductor exhibits a positive charge on the surface. Unless otherwise noted, the present disclosure addresses negative charging photoconductors. Conventionally, the charge generation layer comprises a polymeric binder containing a charge generating compound or molecule while the charge transport layer comprises a polymeric binder containing a charge transport compound or molecule. The charge generating compounds within the charge generation layer are sensitive to image-forming radiation and photogenerate free electron-hole pairs within the charge generation layer as a result of such radiation. The charge transport layer is usually non-absorbent of the image-forming radiation and the charge transport compounds serve to transport holes to the surface of the photoconductor.
One problem associated with some organic photoconductors is that their wear performance is generally inferior to that of inorganic photoconductors, such as amorphous silicon. Photoreceptor wear in the print area is either roughly uniform or non-uniform in nature. This latter wear mechanism often appears as gouges or scratches on the photoreceptor surface, which may manifest themselves as defects in the printed product. Even thin scratches can result in a general print lightning when present in a sufficient density. Concurrent with the appearance of scratches is a general abrasion of the charge transport layer (or charge generation layer when charging positively). The thickness loss changes the electrical properties of the photoreceptor by (1) changing the capacitance and (2) decreasing the transit time required for holes to discharge the surface. This abrasive wear results from interaction of the photoreceptor with other EP components such as toner, cleaner blade and paper.
Providing photoreceptors with a high degree of electrical stability over cartridge life is one goal of the photoreceptor development community. Another important goal is to provide photoreceptors with exceptionally long lives such as greater than 100 k prints. Heretofore it has been difficult to develop a photoreceptor which can meet these two goals simultaneously. One tried route to achieving both of these goals is the application of a crosslinkable protective coating as the outermost layer of the photoreceptor. However, abrasion resistant crosslinkable monomers, oligomers and polymers are generally electrically insulating. Photoreceptors must be insulating in the dark, but conductive when exposed to light. Depositing an insulating layer on the surface of the photoreceptor prevents charge migration, and thus prevents generation of the electrical contrast required to tone an image. One way of combining both high abrasion resistance and proper electrical properties includes forming overcoats comprising charge transport molecules further comprising crosslinkable functional groups. The resulting crosslinked layer, typically 3-5 μm thick, is highly resistant to thickness loss and therefore demonstrates much greater electrical stability than a non-overcoated photoreceptor. A photoreceptor overcoat comprising a UV crosslinked layer of hexacoordinate urethane acrylate and UV crosslinkable charge transport molecule is disclosed in U.S. patent application Ser. No. 13/731,594 entitled “Photo Conductor Overcoats Comprising Radical Polymerizable Charge Transport Molecules and Hexa-Functional Urethane Acrylates”, which is assigned to the assignee of the present application and is incorporated by reference herein in its entirety.
The most commonly used charge transport molecule used in modern laser printers are triarylamines. Production of arylamine hole transport compounds require the synthesis of intermediate materials, which are costly and/or time-consuming to produce, and some of which require a multi-step manufacturing process. For example, diarylamines may be produced using traditional Goldberg reactions. This method requires the derivatization of an aniline with acetic anhydride to produce an acetanilide compound. The acetanilide compound is then reacted with an arylbromide compound to produce an intermediate that must then be hydrolyzed in alcohol solution to produce the diarylamine compound. The formation of diphenylamines using the Goldberg reaction takes three reaction steps, and thus can be a lengthy process. Total cycle time for this process can be 3 to 5 days in the lab. Diarylamines may also be produced by subjecting an arylamine to condensation reaction in the co-presence of anhydrous aluminum chloride and anhydrous calcium chloride. Both of these methods require high temperatures and harsh reaction conditions. The purity of the diarylamines obtained from these two reactions are generally low, requiring lengthy and costly purification processes. The resulting diarylamines may be reacted with halogenated aryl compounds to form a variety of triarylamine compounds.
Until recently, the most common method of producing triphenylamines was the Ullman reaction. Ullman coupling chemistry has been known for over one hundred years and generally involves reaction of an amine with an aryl iodide in the presence of copper. The Ullman reaction suffers from the following drawbacks: (1) aryl iodide compounds are generally more expensive than their bromide or chloride analogues, (2) the reaction uses large, sometimes stoichiometric, amounts of copper and is thus not environmentally friendly, (3) the reaction temperature is often very high and thus requires use of high boiling and hazardous solvents such as dichlorobenzene and high boiling solvents are notoriously difficult to remove, (4) the reaction often produces side products that may be difficult to remove by standard purification methods: and (5) Ullman chemistry does not allow the presence of protic functional groups such as alcohols.
The last 20 years have seen the development of an alternative to the Goldberg and Ullman reactions. The Buchwald-Hartwig reaction allows for production of di- and triarylamine compounds by coupling an arylamine with an aryl halide in the presence of a ligated palladium catalyst and base. Buchwald-Hartwig chemistry has at least four advantages over the Goldberg and Ullman reactions discussed above. (1) The process allows for use of aryl bromide and chloride reagents and thus represents an immediate cost reduction. (2) Very low catalyst loadings, sometimes as low as 0.05%, are required and may thus be viewed as environmentally friendly. (3) The reaction is performed in relatively low boiling solvents such as THF or toluene that are easily removed by vacuum distillation. (4) The reaction provides both high yields and high purity.
Formation of triarylamines comprising at least one alkyl hydroxy group according to the methods cited above requires protection of the hydroxy group(s). The hydroxy group is the most common precursor for incorporation of crosslinkable groups, such as acrylates, to molecules. Protection is required since alcohols, as well as other protic functional groups, can interfere with the triphenylamine synthesis. Alcohol protection adds two synthesis/purification steps (protection and deprotection) to the synthesis and is thus unproductive.
In a general sense, the photoreceptor is called upon to create (with the image writing light source), develop, and transfer a latent image to a substrate. In large part, the electrical changes that occur to a photoreceptor throughout print life are a result of charge transport layer thickness changes induced by abrasion with other components of the EP system. Addition of a crosslinkable overcoat layer that combines excellent wear resistance with the semi conductive properties of a photoreceptor greatly increases the useful life of the photoreceptor while also improving the electrical stability. Current methods for preparing charge transport molecules, such as triarylamines, incorporating alkyl alcohols are inefficient, time consuming, and costly. The primary reason for the deficiency in making triarylamines including alkyl alcohols is the need to include the necessary protection and deprotection steps to the synthesis of triarylamines incorporating alkyl alcohols. Using the alkyl alcohol as a starting material, crosslinkable functionality may be incorporated using well known methods such as reacting the alkyl alcohol with acryloyl chloride in the presence of a base such as diethylamine. Consequently, an efficient method for producing triarylamines incorporating alkyl alcohols remains an unmet need within the electrophotographic arts.